In recent years, the signal band of information communication equipment, such as a PHS or a mobile telephone, and the CPU clock time of a computer have reached the GHz band. The trend toward higher frequencies has been increasingly growing. The transmission loss of an electric signal is represented by the sum of a dielectric loss, a conductor loss, and a radiation loss. The higher the frequency of the electric signal, the more dielectric loss, the conductor loss, and the radiation loss. Since the transmission loss attenuates the electric signal and degrades the reliability of the electric signal, it is necessary to take any measure to suppress increases in dielectric loss, conductor loss, and radiation loss in a wiring board which handles a high-frequency signal. The dielectric loss is directly proportional to the square root of the dielectric constant of an insulator forming a circuit, and to the product of the dielectric loss tangent thereof and the frequency of a signal in use. Accordingly, an increase in dielectric loss can be suppressed by selectively determining an insulating material having a low dielectric constant and a low dielectric loss tangent as the insulator.
Representative materials having low dielectric constants and low dielectric loss tangents will be shown below. A fluorine resin typically represented by polytetrafluoroethylene (PTE) is low in each of dielectric constant and dielectric loss tangent, so that it has been used previously for a substitute material which handles a high-frequency signal. On the other hand, various non-fluorine insulating materials having low dielectric constants and low dielectric loss tangents have also been studied. Because they are readily rendered as varnishes with organic solvents, and they are low in each of molding temperature and curing temperature, and easy to handle. For example, Japanese Patent Laid-Open No. Hei 08-208856 discloses that a diene polymer such as polybutadiene is impregnated into glass cloth and cured with peroxide. Japanese Patent Laid-Open No. Hei 10-158337 discloses an example of annular polyolefin prepared by introducing an epoxy group into a norbornene containing polymer and imparting curability thereto. Japanese Patent Laid-Open No. Hei 11-124491 discloses that acyanate ester, a diene polymer, and an epoxy resin are rendered into the B-stage by heating. Japanese Patent Laid-Open No. Hei 09-118759 discloses an example of a modified resin made of polyphenylene oxide, a diene polymer, and triallyl isocyanate.
Japanese patent Laid-Open No. Hei 09-246429 discloses an example of a resin composition made of allylated polyphenylene ether, triallyl isocyanate, and the like. Japanese Patent Laid-Open No. Hei 05-156159 discloses that polyetherimide and styrene, divinylbenzene, or divinylnaphthalene are alloyed. Japanese Patent Laid-Open No. Hei 05-078552 discloses an example of a resin composition made of, e.g., bis(vinylbenzyl)ether and a novolac phenol resin; the resin composition has been synthesized from a dihydroxy compound and chloromethylstyrene through the Williamson reaction. Japanese Patent Laid-Open No. 2002-249531, Japanese Patent Laid-Open No. 2003-012710, and Japanese Patent Laid-Open No. 2003-105036 disclose that a multifunctional styrene compound having a total hydrocarbon skeleton is used as a crosslinkable component.
Furthermore, prior arts disclose that a filler, a flame-retardant agent, and the like, each having a low dielectric constant and a low dielectric loss tangent is added to the above mentioned insulating materials having low dielectric constants and low dielectric loss tangents, thereby further improving the dielectric properties and imparting a property such flame retardancy thereto. For example, Japanese Patent Laid-Open No. 2003-342311 discloses such an approach which combines 1,2-bis(pentabromophenyl)ethane, 1,2-bis(tetrabromophthalimide)ethane, and a multifunctional styrene compound. Japanese Patent Laid-Open No. 2000-043041, Japanese Patent Laid-Open No. 2000-043042 and Japanese Patent Laid-Open No. 2000-198113 disclose that 1,2-bis(pentabromophenyl)ethane is added to a curable polyphenylene ether resin composition as such an approach. Japanese patent Laid-Open No. Hei 09-031275 discloses that 1,2-bis(pentabromophenyl)ethane is added to polystyrene.
An object of the present invention is to impart stability to a low dielectric loss tangent-resin varnish used in the process of manufacturing a prepreg, for the purpose of improving the productivity of a printed wiring board or multilayer printed wiring board with a low dielectric constant and a low dielectric loss tangent. Prior arts disclose that an insoluble component such as a filler or flame-retardant agent is added to a low dielectric loss tangent resin composition. By the way, when the low dielectric loss tangent resin composition is rendered as a varnish, precipitation of the insoluble component may occur. In prior arts, sufficient measures have not been made against the precipitation of the insoluble component. The present invention intends to cope with such a problem.
Another object of the present invention is to provide a low dielectric loss tangent-resin varnish having excellent storage stability and being capable of providing a cured product exhibiting excellent dielectric properties in a high-frequency range. Furthermore the present invention is to provide a wiring board member using the same, such as a prepreg, a laminated board, a printed wiring board, or a multilayer printed wiring board.